To study the development of tissue and cell damage, the early morphologic changes induced by photodynamic therapy (PDT) with the new photosensitizer bacteriochlorin a (BCA) were investigated in Greene hamster melanoma implanted in the anterior eye chamber of white rabbits up to 24 hr after BCA-PDT, using light and electron microscopy. Immediately after BCA-PDT, intracellular spaces were enlarged, and blood vessels were clotted with swollen erythrocytes. By electron microscopy, it was found that some mitochondria had fused inner and outer membranes, and the cristae mitochondriales were affected. With time, the severity of the tissue and cell damage increased, leading to almost complete tumor necrosis after 24 hr. The direct mitochondrial damage and the vascular damage induced by BCA-PDT probably both contribute to tumor necrosis.